Transcript Scene 1 - Cloudfront.net

Scene 1: Michelle is saved by the
starving bacteria
After this lesson, the student will be able to:
1) explain recessive inheritance using phenylketonuria (PKU) as
an example;
2) describe the Guthrie test for screening newborn babies for
PKU
3) explain how PKU can be treated by a special diet and how this
could postpone the effects of PKU until the next generation
4) discuss some of the economic issues associated with
screening for the treatment of genetic conditions
5) explain why females with PKU need to be followed very closely
in their reproductive years to avoid the problems associated
with maternal PKU.
Michelle’s Situation
Stacy and Paul Chance
are referred to the
genetics center
because their newborn
daughter, Michelle, had
elevated phenylalanine
levels as determined
by newborn screening.
The diagnosis of
phenylketonuria (PKU)
is made.
Mandatory PKU Testing
The couple wonders why their
child was tested. They have
two other children, Alan (age 6)
and Frank (12) who are
apparently healthy, active,
intelligent boys. The couple
couldn't remember any special
testing that either boy had at
birth. State mandated newborn
screening is explained to them.
A description of PKU and its
inheritance is provided, and
dietary management is
discussed.
PKU Testing: Guthrie Test
• Mandatory statewide
testing around day 2
($5/child)
• Bacteria grows on
blood sample if
excess phenylalanine
is present
• Treatment begins
immediately
• Why is $5/child more
cost effective for the
state then no testing
at all?
What is PKU?
• PHENYLKETONURIA (PKU)
– PAH gene on long arm of chromosome
12 is absent/reduced
– Excess phenylalanine interferes with
brain development
• Autosomal recessive disorder
1:10,000-15,000 births
• Effects: retardation, seizures, destructive,
restlessness
Prognosis
• With treatment:
Normal development
• With minimal
treatment: Intellectual
defects
• Without treatment:
Retardation
Treatments
• Low phenylalanine diet
• Baby fed special
formula
(~$10,000/year)
• Vegetables and low
phenylalanine foods
added later in
development
• Regular blood samples
taken and
phenylalanine diet
adjusted as needed
• Eventually, a normal
diet can be obtained
Chemistry of PKU
• Normally, phenylalanine is converted into tyrosine and
then into a protein
• Enzyme phenylalanine hydroxylase is absent/reduced
• Phenylalanine builds up in blood
Chances’ reaction
Paul denies his contribution to his daughter's condition.
He is a successful businessman, proud of his healthful
lifestyle. He doesn't smoke, he jogs six miles a day
and he enjoys a healthy diet. He attributes Michelle's
condition to a "weakness on Stacy's side of the family"
and refuses to believe that he has any "bad genes."
The genetic counselor mentions that everyone carries
an estimated 6-10 "unusual" recessive genes which
cause no clinical problems in the carriers, but if two
carriers of the same unusual recessive gene were to
have children together, each child would have a 1 in 4
chance of inheriting the recessive condition for which
the genes were responsible.
Genetic Counseling
• Paul is relieved to know that being a
carrier neither reflects negatively on his
manhood nor does it isolate him from his
peers. He quickly turns the situation to his
advantage and states, "Well, if we're all
carriers of some bad genes, I guess I'm
better off than most of you. At least I know
what one of my unusual genes is." Stacy
does not try to defend herself against
Paul's earlier accusations but is relieved at
his acceptance of the cause of Michelle's
condition.
• The further complication of maternal PKU is mentioned. Paul and
Stacy are scheduled to meet with a nutritionist, and follow-up
appointments for monitoring blood phenylalanine levels are made.
• Upon leaving, Stacy thanks the genetics staff and says, "Well I guess
we won't be seeing you again for a long time."
• Little does she know...
Essay Topic #1
Assume that you are
Stacy or Paul Chance.
Describe in writing
your feelings after your
visit to the genetics
center.
•
Essay Format:
– Ink or typed (no pencil)
– Add title and date
– Minimal length = ¾ page
– If typed: 1” margins, 12-point times new roman
font, double spaced.
– Complete essays (Introduction, body,
conclusion)
– Spellcheck and proofread